Searching for gravitational wave bursts with the new global detector network 2007 May 3 Searching for gravitational wave bursts with the new global detector network Shourov K. Chatterji INFN Sezioni di Roma / Caltech LIGO Laboratory for the LIGO Scientific and Virgo Collaborations 7th Eduardo Amaldi Conference on Gravitational Waves 2007 July 8-14 Sydney, Australia Shourov K. Chatterji - EGO Seminar - Cascina, Italy

Abstract EGO Seminar - 2007 May 3

Overview Benefits of joint data analysis Agreement on joint data analysis Previous studies based on coincidence methods Investigations with simulated data Current investigations with real data Coherent methods for joint data analysis Special case of collocated detectors General case of multiple non-aligned detectors Maximum likelihood statistics Null stream consistency tests Source reconstruction Hierarchical search approach Conclusions EGO Seminar - 2007 May 3

Benefits of a global network Improved sky coverage Less likely that event occurs in null of detector network Improved duty cycle More likely that at least one detector observes an event EGO Seminar - 2007 May 3

Benefits of a global network Increased signal to noise ratio Coherently sum signals from multiple detectors Improved detection confidence Multi-detector coincidence greatly reduces false rate Coherent consistency tests can differentiate between gravitational-wave signals and instrumental glitches Improved source reconstruction “Inverse problem” requires three non-aligned detectors Provides sky position and both polarizations of waveform Permits comparison with theory This is where the science is! Shared best practices Learn from each other’s approaches EGO Seminar - 2007 May 3

Agreement on joint data analysis Recognizing the benefits of joint data analysis, the LIGO Scientific and Virgo collaborations have entered into an agreement to jointly analyze data from the GEO, LIGO, and Virgo detectors Sharing of data started in May 2007 when Virgo commenced its first long science run in coordination with LIGO’s fifth science run A joint run plan committee is coordinating detector operation and commissioning schedules to improve the prospects for detection Joint data analysis meetings are already taking place Joint data analysis exercises with simulated and small amounts of real data have already been performed for the inspiral, burst, and stochastic analysis EGO Seminar - 2007 May 3

Project 2b ~72 hours of coincident data from the G1, H1, H2, L1, and V1 detectors from September 2006 (LIGO’s fifth science run and Virgo’s first weekly science run) Data shifted in time by a secret amount Perform “end to end” analysis using one of the methods Produce upper limits for a variety of simulated burst signals Study coherent methods Identify options and issues for the upcoming joint analysis 1.2 days 1.4 days 1.89 days 2.1 days 2.2 days 1.8 days H1 V1 L1 EGO Seminar - 2007 May 3

Coincident analysis Applied the excess power search algorithm Searches for statistically significant signals using a time-frequency decomposition of the data Tests for coincident events between multiple detectors Searched from 800 to 2000 Hz EGO Seminar - 2007 May 3

Pairwise coincident sensivity Searching for gravitational wave bursts with the new global detector network 2007 May 3 Pairwise coincident sensivity Based on previous investigations, we study the union of all pairwise coincident triggers The detection threshold in each detector was set to maximize detection efficiency for all simulated waveforms at a false event rate of 1 uHz. The resulting efficiencies are compared with the efficiency of using only the two 4km LIGO detectors. EGO Seminar - 2007 May 3 Shourov K. Chatterji - EGO Seminar - Cascina, Italy

Coherent analysis Naturally handles arbitrary networks of detectors Coherent sum: Find linear combinations of detector data that maximize signal to noise ratio data = response x signal + noise X = F h + n coherent null N-2 dimensional null space detector data Null sum: Linear combinations of detector data that cancel the signal provide useful consistency tests. coherent sum 2 dimensional signal space EGO Seminar - 2007 May 3

Coherent analysis Applied the Coherent WaveBurst search algorithm Considered the performance of different network configurations Studied the frequency band from 256 to 2048 Hz Both the sensitivity and the stationarity of a detector impacts the detection efficiency of the coherent waveburst anlaysis Signal amplitude required for 50 percent detection efficiency network hrss@50% sg361q9 sg849q9 sg1615q9 live time sec H1xH2 11x10-22 16x10-22 31x10-22 182772 L1xH1xH2 8x10-22 14x10-22 37x10-22 157599 L1xH1xH2xV1 9x10-22 17x10-22 40x10-22 104062 L1xH1xH2xG1 41x10-22 140351 L1xH1xH2xV1xG1 42x10-22 102907 EGO Seminar - 2007 May 3

Comparison of detection efficiencies Searching for gravitational wave bursts with the new global detector network 2007 May 3 Comparison of detection efficiencies Detection efficiencies of pairwise coincident search shows good performance relative to coherent search from 800 to 2k Hz Potentially due to larger frequency range of coherent search or effect of non-stationarities on coherent search EGO Seminar - 2007 May 3 Shourov K. Chatterji - EGO Seminar - Cascina, Italy

Note The following slides on coherent consistency testing refer to work that is not yet complete. If results are available in time to be approved for Amaldi, they will be included here. If not, these simulated results will be used to illustrate the method.

Coherent consistency tests Coherent methods also off the possibility of consistency tests Test null stream for consistency with noise over sky c2 / DOF consistency with a GWB as a function of direction for a simulated supernova (~1 kpc) Interference fringes from combining signal in two detectors. True source location: intersection of fringes c2 / DOF ~ 1 GWB: Dimmelmeier et al. A1B3G3 waveform, Astron. Astrophys. 393 523 (2002) , SNR = 20 Network: H1-L1-Virgo, design sensitivity EGO Seminar - 2007 May 3

Example consistency sky maps Simulated gravitational-wave burst Simulated coincident glitch EGO Seminar - 2007 May 3

Source position recovery Given a statistically significant and consistent signal What direction on the sky maximizes the coherent sum? What direction on the sky minimizes the null stream? Sky position estimates typically fall along rings of constant time delay between sites, leading to poor direction. Other statistics or image processing techniques may be needed Null energy Enull varies slowly along rings of constant time delay with respect to any detector pair. Noise fluctuations often move minimum away from source location (pointing error) source location EGO Seminar - 2007 May 3

Coherent sky position recovery example Comparison of sky position recovery by Coherent WaveBurst algorithm for LIGO only and LIGO/GEO/Virgo networks. H1xL1 L1xH1xH2xV1xG1 EGO Seminar - 2007 May 3

Waveform recovery Given a statistically significant and consistent signal and a best guess sky position Solve the matrix equation x = Fh + n for the waveforms h Find the psuedo-inverse R such that RF = I Network: H1-L1-GEO GWB: Zwerger-Muller A4B1G4, SNR=40 [Astron. Astrophys. 320 209 (1997)] Recovered signal (blue) is a noisy, band-passed version of simulated GWB signal (red) Injected GWB signal has hx = 0. Recovered hx (green) is just noise. EGO Seminar - 2007 May 3

Summary Coincident and coherent analysis tools are now available to take advantage of data from networks of detectors The union of pairwise coincident triggers from the LIGO/Virgo network provides an improvement in detection efficiency and coverage over the LIGO only network Coherent methods naturally handle arbitrary networks of detectors and provide increased detection confidence, consistency tests, and parameter estimation. Coincident and coherent analysis methods complement eachother and can be used as part of a hierarchical search We are ready to start analyzing joint GEO, LIGO, and Virgo data EGO Seminar - 2007 May 3

Outlook Most recent official detector noise spectrums will go here Tentative timeline for detector operations including enhanced and dvanced detectors will go here. EGO Seminar - 2007 May 3

End

Initial joint working group Need to plan for joint analysis as soon as possible First meeting held in June 2004 Investigations using simulated detector noise Develop common language to describe searches Learn from and gain confidence in search algorithms Demonstrate ability to analyze each other’s data Confront different sample frequencies, file formats, etc. Quantify the benefit of the LIGO/Virgo network Investigations using real detector noise Learn to handle data quality and veto information Confront non-ideal behavior of real detectors Identify options for future joint analysis EGO Seminar - 2007 May 3

Project 1A 3 hours of simulated H1 and V1 detector noise A variety of simulated waveforms: Binary neutron star inspirals Unmodeled Bursts: Core-collapse supernovae Gaussians Sinusoidal Gaussians Applied a variety of LSC and Virgo search algorithms Characterized and compared search algorithm performance Burst and inspiral results presented at GWDAW 9 and published in corresponding conference proceedings Burst: Class. Quant. Grav. 22, S1293 (2005) Inspiral: Class. Quant. Grav. 22, S1149 (2005) EGO Seminar - 2007 May 3

Project 1A burst results EGO Seminar - 2007 May 3

Project 1B 24 hours of simulated triple coincident (H1, L1, and V1) data Injection populations of simulated waveforms Binary neutron star inspirals from M87 and NGC 6744 Supernovae in the direction of the galactic center Demonstrated benefit of union of two detector networks Demonstrated sky position reconstruction and astrophysical parameter estimation Joint burst and inspiral results presented at Amaldi 6 and published in corresponding conference proceedings J. Phys. Conf. Ser. 32, 212 (2006) EGO Seminar - 2007 May 3

Project 1b results For coincident searches, union of double coincident detector networks provides improved performance Burst results for simulated supernovae in the direction of the galactic center at a 1 mHz false rate: Inspiral results for simulated signals from M87 and NGC 6744 at an SNR threshold of 6: EGO Seminar - 2007 May 3

Timing based recovery of sky position 1 ms Gaussian signals recovered within ~1 degree of true position using Gaussian matched filter and timing based recovery of position Rings of constant time of arrival delay between detectors Source location Plane of detectors Mirror image source location EGO Seminar - 2007 May 3

Project 1 results Comprehensive papers describing work on simulated data Expand on previous work incorporating lessons learned Burst: arXiV:gr-qc/0701026 Investigate robustness over large signal space Investigate intersection and union of algorithms Inspiral: arXiV:gr-qc/0701027 Detailed comparison of analyses pipelines Bayesian estimation of source parameters EGO Seminar - 2007 May 3

Project 2a First exchange of real data occurred on 2006 June 15 ~3 hours of non-coincident G1, H1, and V1 data including: Periods of lock loss Periods of good and bad data quality Hardware injections of burst and inspiral waveforms Data quality and veto segment information also exchanged Applied LSC and Virgo search algorithms Results shared at a working group meeting on 2006 June 23-24 in Orsay, France EGO Seminar - 2007 May 3

Project 2b ~72 hours of coincident data from the G1, H1, H2, L1, and V1 detectors from September 2006 (LIGO’s fifth science run and Virgo’s first weekly science run) Data shifted in time by a secret amount Perform “end to end” analysis using one of the methods Produce upper limits for a variety of simulated burst signals Study coherent methods Identify options and issues for the upcoming joint analysis Results to be presented at Amaldi7 in Sydney, Australia EGO Seminar - 2007 May 3

Coincident and coherent analysis Coincident methods Test for time and/or frequency coincidence between detectors Performance constrained by least sensitive detector Difficult to select detector dependent significant thresholds for optimal performance Coherent methods Test linear combinations of detector data based on: Frequency dependent sensitivity of each detector Expected response of each detector to gravitational waves from a particular direction on the sky Naturally handles arbitrary numbers of detectors Naturally handles detectors with different sensitivities Three different applications of coherent methods: Event detection Consistency testing Source reconstruction EGO Seminar - 2007 May 3

General case of non-aligned detectors Y. Gursel & M. Tinto PRD 40 3884 (1989) “Near Optimal Solution to the Inverse Problem for GWBs”. Full solution requires at least three non-aligned detectors First solution of the problem for the three detector case Procedure: Use a linear combination of time shifted signals from two detectors to predict the signal in the third detector For each direction on the sky, compute the prediction error Find the direction on the sky where the prediction error is minimized Yields best fit sky position Also yields both polarizations of gravitational waveforms This is equivalent to the maximum likelihood approach EGO Seminar - 2007 May 3